Heart rate is widely accepted as a good method for measuring intensity during running, swimming, cycling, and other aerobic activities. Exercise that doesn’t raise your heart rate to a certain level and keep it there for 20 minutes won’t contribute significantly to cardiovascular fitness.
The heart rate you should maintain is called your target heart rate. There are several ways of arriving at this figure. One of the simplest is: maximum heart rate (220 – age) x 70%. Thus, the target heart rate for a 40 year-old would be 126. Beginners should maintain the 60 percent level, more advanced can work up to the 80 percent level. This is just a guide however, and people with any medical limitations should discuss this formula with their physician.
You can do different types of aerobic activities, say walking one day, riding a bike the next. Make sure you choose an activity that can be done regularly, and is enjoyable for you. The important thing to remember is not to skip too many days between workouts or fitness benefits will be lost. If you must lose a few days, gradually work back into your routine.